1. Field of the Invention
The present invention relates to automatic alignment, and more particularly to a method and an apparatus for detecting the alignment of objects wherein a laser beam scans the objects to be aligned and is reflected by alignment marks to produce scattered light which is used for alignment, during production of semiconductor integrated circuits.
2. Description of the Prior Art
In this type of device, the practice has been to dispose a semiconductor wafer and a mask one above the other and scan their alignment marks to detect the position of the marks. For a better understanding of such a system, U.S. Pat. Nos. 4,167,677 and 4,199,219 and here referred to. The mask has an alignment mark, as shown in FIG. 1, (a), having first and second bar-like mark elements M1 and M2 which are spaced and parallel with each other but inclined (.theta.) with respect to the scanning line A, and also having third and fourth bar-like mark elements M3 and M4 which are spaced and parallel with each other but inclined (.theta.) oppositely with respect to the scanning line A. The wafer has an alignment mark, as shown in FIG. 2, (b), having first and second bar-like mark elements W1 and W2 which are inclined with respect to the scanning line A at the same angle .theta. but in different directions. Those marks of the mask and wafer are superimposed as shown in FIG. 1 (c) to achieve alignment therebetween. When the marks in the state of FIG. 1, (c) are scanned by a laser beam spot along the scanning line A, the beam is scatteredly reflected by the mark elements M1, M2, M3, M4, W1 and W2. The scattered light is received by a photoreceptor which produces pulse signals in response to the positions of the respective mark elements, as shown in FIG. 1, (d). The pulse signals are reformed into rectangular pulses by cutting the tops of the pulse signals at a suitable threshold V with the aid of a comparator. The reformed pulses are shown in FIG. 1, (e). The positional relationship between the wafer and the mask can be obtained from the time intervals between the mark elements. The wafer or mask is displaced in response to the positional relationship thus obtained so as to provide alignment therebetween.
Generally, scanning is effected by a spot of the laser beam. With a spot beam, erroneous or less accurate alignment can occur, when a piece of dust or an aluminium particle is present on the scanning line. In the case of semiconductor wafers in the manufacture of integrated circuits, alignment is required to a degree within microns or even to a degree within sub-microns. So in this field, the alignment is required to be of ultimate accuracy.
After the positions of the alignment marks are detected, the alignment drive is effected to the wafer and/or mask. It should be noted that it is seldom that one drive results in a correct alignment, usually the alignment operation is repeated several times. Also, in order to enhance the accuracy of the detection, the alignment marks are scanned several times to take an average result. Further, in the case of a step and repeat type aligner wherein the alignment operation is carried out for each shot, the alignment operation is executed tens of times for one wafer. Therefore, reduction of the time required for the alignment is highly desired in addition to the accuracy thereof.